dts: arm: stm32H5 reg definition for the st,stm32-xspi compatible

The st,stm32-xspi compatible is defining the reg property
with the register address and size at first index
followed by the external memory base address and max allocated
xspi1 is addressing max 256 MBytes from 0x90000000

Signed-off-by: Francois Ramu <francois.ramu@st.com>